The present invention generally relates to a fixing apparatus for use in an image forming apparatus such as a copy machine or a printer.
Conventionally, as one example of a fixing apparatus as referred to above, there has been proposed, for example, in U.S. Pat. No. 4,385,826, an arrangement which includes a heat roller having a heater and a pressure roller contacting the heat roller under pressure so that toner of a unfixed toner image held on a sheet is melted and fixed on the sheet while the sheet is held by the heat roller and the pressure roller while being transported thereby.
The apparatus generally comprises a temperature detecting element so as to detect the temperature state of the heat roller to control the heater. For prevention of a defective fixing of a copy paper, after the heater turns on, when the element detects that the temperature of the heat roller reaches a specified fixing, pre-rotation start temperature, the heat roller is driven to rotate so as to transmit the heat thereof to the pressure roller and so as to sufficiently heat the toner from both sides of the sheet.
In the conventional fixing apparatus, however, since the temperature detecting element always contacts the heat roller under pressure, there have been involved such problems that the contact portion, contacting the temperature detecting element, of the heat roller is greatly damaged and thus the longevity of the heat roller is reduced.
Furthermore, an essential object of the temperature detecting element is to control the heater in order to maintain the temperature of the heat roller and the pressure roller in the toner fixing operation state. In accomplishing the object, the temperature region of the temperature detecting element should be a comparative narrow region. However, in the above apparatus, the temperature region of the temperature detecting element to detect the temperature thereof is required to be wide region i.e. from a low temperature to a high temperature, for detecting the specified fixing pre-rotation start temperature.